style_selection() should preserve trailing newline

[hadley]

Given

```{r}
1+2
```

If I select 1+2 and then style the current selection, I get

```{r}
1+2```

[lorenzwalthert]

Thanks. I think the problem described only occurs when the whole line is selected with the Addin (see #520). However, I think that we should also get this case right since I imagine it's common to select code like this if one uses the Addin, so we should resolve this in #520.

Side note: For styling a whole .Rmd document, there is support via both styler::stlyle_file(...) and style_pkg(..., filetype = ".Rmd") as well as through the style active file Addin (I guess you knew this already, just for the record).